1. Field of the Invention
This invention relates to an embedded electroconductive layer and a method for the formation thereof. It relates more particularly to an embedded interconnection layer using Cu and Al or a connecting layer (plug) for joining an upper and a lower interconnection layer and a method for the formation thereof.
2. Description of the Prior Art
For the purpose of lowering the resistance offered by a interconnection layer, the feasibility of substituting a Cu interconnection layer for the conventional Al interconnection layer has been the subject of a diligent study. Cu has lower resistivity than Al and about twice as high electromigration resistance as Al.
Generally, the formation of a layer of fine wiring lines requires to resort to dry etching.
Cu, however, encounters the problem that the conventional RIE (reactive ion etching) technique fails to afford an ample etching rate at low temperatures because a halogenide of Cu has a low vapor pressure. It also has the problem of rendering anisotropic etching difficult.
The feasibility of a method which is called damascene and uses the self-aligning technique for the purpose of solving such problems as mentioned above is now being studied.
The damascene method resides in forming an embedded interconnection layer as follows. First, a Cu layer is deposited in grooves and contact holes formed in an insulating layer in conformity with a wiring pattern. Then, the unwanted parts of the Cu layer that protrude from the grooves and the contact holes are removed by chemical mechanical polishing (CMP).
Among the methods available at all for the deposition of Cu in this case is counted the CVD method that excels in step coverage. The combination of the sputtering method deficient in step coverage with the reflow method is also available. Of these two methods, the former CVD (chemical vapor deposition) method is expected to grow into a prospective technique for the formation of a Cu interconnection layer fit for future trend toward increasing fineness of circuitry.
Further, Cu easily diffuses in SiO.sub.2, forms a deep sublevel in a silicon semiconductor, and curtails the service life of minority carriers. In the formation of a Cu interconnection layer by the damascene method, a barrier metal layer such as the TiN layer is formed on an SiO.sub.2 layer and a Cu layer is grown thereon for the purpose of preventing Cu from diffusing in the SiO.sub.2 ("Advanced Metallization for ULSI Application," compiled by V. S. Rana et al., written by J. A. T. Norman, D. A. Roberts, A. K. Hochberg, and R. Laxman, and published by MRS, Pittsburgh, 1993).
When a Cu layer is grown by the CVD method on such a CVD-TIN layer as mentioned above, an anneal performed in the neighborhood of 600.degree. C. tends to induce an alloying reaction between Cu and the barrier metal, though the incubation time between the time the step of deposition starts and the time the actual deposition of film begins decreases in proportion as the degree of oxidation on the surface of the barrier layer decreases. The growth of the Cu layer under discussion, therefore, encounters the problem that the CVD-TiN layer does not function as a barrier metal.
When a TiN layer which has a higher oxygen concentration than the CVD-TiN layer and is formed by the sputtering technique (hereinafter referred to as "PVD (physical vapor deposition)-TiN layer") is used for the purpose of exalting the barrier property, the problem ensues that the incubation time is elongated.
The cause for the elongation of the incubation time is as follows. It has been reported (S. Cohen et al., Appl. Phys. Lett., Vol. 60(1), Jan. 6, 1992 p.50-52) that during the deposition of a Cu layer using hexafluoroacetylacetonate trimethylvinyl silane Cu:Cu (hfac) TMVS as a precursor, for example, the precursor is caused to undergo decomposition and precipitate Cu owing to the donation of electrons from the surface of the ground layer. When the barrier layer constituting a ground layer has a high oxygen concentration and exhibits low metallicity, therefore, the incubation time is suffered to elongate because the donation of electrons from the surface of the barrier layer does not easily occur.
When an embedded electroconductive layer is formed by having an insulating film such as of copper embedded through the medium of a barrier layer in a groove as described above, this barrier layer is demanded to fulfill such mutually contradicting requirements as enhancing the barrier property of the barrier layer and curtailing the incubation time.